Conventional protection devices for pipelines are provided with a coupled gas-liquid emergency cut-off system. When a signal is generated indicative of a pressure drop caused by a pipe rupture, the system utilizes a plurality of valves and energy accumulating tanks to multistagedly transmit, transform and amplify the signal to generate a thrust large enough to push a hydraulic driving device so as to drive a ball valve to cut-off the flow path. Such a pipeline protection system has a complicated structure and a long signal transmission route, and it is therefore necessary to utilize external forces generated by energy cumulating tanks and the like to achieve the protection of pipelines.